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Australia has ten stocks of Blue Swimmer Crab across WA, Qld, NSW and SA. Eight of those stocks are sustainable with one stock in WA classified as depleted and one stock In South Eastern Australia classified as depleting.

Blue Swimmer Crab is distributed in Australia from the south coast of Western Australia, north to the Northern Territory, across Queensland, down the east coast and to the New South Wales–Victoria border. They are also found in the warmer waters of the South Australian gulfs [Kailola et al. 1993].

In Western Australia, Blue Swimmer Crab is fished in numerous fisheries across five regions. The stock delineation between these regions is unknown [Chaplin et al. 2001; Chaplin et al. 2008]. Stock structure on the east coast of Australia is uncertain, involving overlapping stocks or a semi-continuous stock [Chaplin et al. 2001]. Using a high-resolution oceanographic model coupled with a Lagrangian particle tracking framework to simulate larval dispersal, Hewitt et al. 2022 suggest populations of Blue Swimmer Crab in NSW and Queensland appear to constitute demographically separate stocks, supporting the current assessment and management at the state level. In South Australia, research has identified three separate biological stocks of Blue Swimmer Crab—in Spencer Gulf, Gulf St Vincent and on the coastline west of the Eyre Peninsula [Bryars and Adams 1999; Dixon and Hooper 2011].

Here, assessment of stock status is presented at the biological stock level—North-Eastern Australia (Queensland), South-Eastern Australia (New South Wales), Spencer Gulf, Gulf St Vincent and West coast (South Australia), and at the management unit level—Shark Bay, Cockburn Sound, Peel-Harvey Estuary, Western Australian North Coast and Western Australian South-West Coast (Western Australia).

North-Eastern Australia

The Queensland Blue Swimmer Crab fishery primarily operates in southern Queensland. Prior to 1998, most fishing was conducted inshore, in and around Moreton Bay. In 1998 commercial pot fishers began exploiting Blue Swimmer Crab populations further offshore, in areas that were previously lightly fished. Fishing in offshore waters peaked in 2003, when the offshore harvest contributed approximately 70% to total harvest. By 2015, the offshore harvest had decreased and returned to levels slightly higher than those pre-expansion. This rise and subsequent fall of harvest in the offshore area may indicate a decline in fishable biomass for the offshore area and the biological stock as a whole [Johnston et al. 2018]. However, proposed management reforms and subsequent over-reporting within the fishery prior to the investment warning released in 2003 likely reduced the reporting reliability of commercial catch records around this time [QDAF 2019].

Queensland assessed the North-Eastern Australia Blue Swimmer Crab stock in 2020 (including data up until the 2018–19 financial year) using an integrated stock assessment model. The model estimated exploitable biomass to be at around 33% relative to unfished levels [Lovett et al. 2020]. Under the current management arrangements (i.e. minimum legal size, no-take females), maximum sustainable yield (MSY) is estimated to be around 722 t [Lovett et al. 2020]. Average combined commercial and recreational harvest from the most recent five years was approximately 297 t (267 and 30 t respectively), less than half of the estimated MSY [Webley et al. 2015; Lovett et al. 2020; Teixeira et al. 2021].

Standardised commercial pot catch rates have been declining since 2003–04 with the 2018–19 fishing year having recorded the lowest catch rate since the introduction of logbooks in 1988–1989 [Lovett et al. 2020]. While standardised catch rates have been low, historical records indicate periods close to this figure (between1992–93 and 1996–97 fishing years). Standardised catch rates are not available for the current period. Declining standardised catch rates may indicate a reduction in the level of biomass for the North-Eastern Australia biological stock.

Fishery independent surveys show that recruitment abundance has generally been stable through time, with two distinct peaks observed in 2019–20 (24 crabs per hectare) and 2020–21 (32 crabs per hectare) [QDAF 2023 Unpublished data, Bessell-Browne et. al. 2020]. The 2020–21 estimate of recruitment abundance is of particular interest as it surpasses the previous high estimate from 2008–09 by more than 65%. Recruitment target reference points are still yet to be established by Fisheries Queensland. The most recent stock assessment [Lovett et al. 2020] estimated that spawning biomass displays similar trends to the exploitable biomass, and in 2018–19 was estimated to be around 50% of 1988–89 levels. This stock is not considered to be recruitment impaired.

Active commercial pot fishing licences and fishing effort (in days fished) decreased between 2003–04 and 2021–22 by approximately 81and 85% respectively. Long term trends in effort are directly proportional to the expansion and subsequent contraction of fishing in offshore areas [Sumpton et al. 2015], as well as the introduction of multiple crab symbols on a licence and vessel tracking. Overall fishing pressure on the North-Eastern Australia Blue Swimmer Crab stock is declining.

Spatial closures within the Moreton Bay, Great Sandy Strait and Great Barrier Reef Marine Parks provide protection of the Blue Swimmer Crab biomass [Johnston et al. 2018]. Management arrangements in Queensland prohibit the take of female Blue Swimmer Crabs, and a minimum legal size of 115 mm carapace width ensures that a high proportion of male Blue Swimmer Crabs have the opportunity to mate before recruitment into the fishery [Johnston et al. 2018]. Further, the recent transition to a quota-managed fishery and the introduction of a Total Allowable Commercial Catch (263 t) as well as a recreational possession limit of 20 Blue Swimmer Crabs per person (from no limit) should further bolster protection of this species. The current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.

On the basis of the evidence provided above, the North-Eastern Australia (Queensland) biological stock is classified as a sustainable stock.

Blue Swimmer Crab Biology [de Lestang et al. 2003a,b; Sumpton et al. 2003]

Blue Swimmer Crab Spatial Distribution

Western Australia – Recreational (Catch). Boat-based recreational catch in 2017–18 [Ryan et al. 2019]. Does not include scoop netting and other methods of recreational fishing.

Queensland – Indigenous (Management Methods). for more information see: https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing

Queensland – Commercial (Catch). Queensland commercial and charter data have been sourced from the commercial fisheries logbook program. Further information available through the Queensland Fisheries Summary Report https://www.daf.qld.gov.au/business-priorities/fisheries/monitoring-research/data/queensland-fisheries-summary-report

Queensland – Commercial (Management Methods). Harvest strategies available at: https://www.daf.qld.gov.au/business-priorities/fisheries/sustainable/harvest-strategy

Queensland – Recreational Fishing (Catch). Data are based at the whole of Queensland level and derived from statewide recreational fishing surveys. Where possible, estimates have been converted to weight (tonnes) using best known conversion multipliers. Conversion factors may display regional or temporal variability. In the absence of an adequate conversion factor, data presented as number of fish.

New South Wales – Indigenous (Management Methods). (https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing)

New South Wales – Recreational (Catch) . Recreational catch estimate of 20 t t is based on (i) an estimated recreational catch of 42,200 Blue Swimmer Crabs by NSW resident recreational anglers in 2019–20 [Murphy et al. 2021]; and (ii) an assumed mean weight of kept Blue Swimmer Crabs of 0.225 kg/crab.

Commercial catch of Blue Swimmer Crab - note confidential catch not shown.

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